In the urban parts of Minneapolis and St. Paul, Minnesota, around three hundred thousand tons of salt are dumped onto roads each winter. That’s because sodium chloride lowers the freezing point of water, making the formation of ice on the surface of roadways less likely. It keeps drivers safer, since car tires can hold onto imperfect road surfaces better than they can grip slippery, wet ice.

But dumping all that salt into the ecosystem doesn’t just keep drivers safer. It also changes the chemical composition of the soil near roadways, and that added sodium chloride finds its way into plants, into bodies of water, and into animals. And all that added salt could shift the dynamics of natural selection and the animals’ fitness, altering the course of evolution. That’s how natural selection works – it’s neither good or bad – but it’s worth at least being aware of the sometimes nuanced effects that human behavior have on natural ecosystems.

Sodium is critical for animal development; it’s a necessary ingredient for proper neural and muscular function. Since it’s a limited resource in most ecosystems sodium cravings evolved along with unique, specific foraging behaviors meant to increase sodium ingestion. For example, some butterflies and bees have evolved to drink the tears of turtles and crocodiles as a means of collecting more sodium. In part, the need for sodium also explains why humans love snack foods so much. Suddenly, lots of animals find themselves in landscapes that now contain an overabundance of sodium, thanks to road salt. What does that mean for their health and their evolution?

Ecologist Emilie C. Snell-Rood and colleagues from the University of Minnesota wanted to understand the affects of high sodium intake on butterfly development. To start, they needed to demonstrate that there was more sodium inside the leaves of plants near salt-covered roads. They measured sodium concentration in four plant species that butterflies eat. Some species soaked up more salt than others. For milkweed and oak, but not for mustard or grass, sodium levels were much higher in plants harvested adjacent to roadways than in plants collected one hundred meters away. Milkweed is critical for monarch butterflies in particular.

Next, Snell-Rood reared monarch caterpillars either on milkweed collected near a salted road or on milkweed collected from farther away. The roadside-collected milkweed had sixteen times more sodium in its leaves than did the control plants. As a result, the male butterflies that were fed the sodium-rich plants had stronger flight muscles, while the females had larger eyes. In other words, male butterflies had more thoracic protein if they were raised on high-sodium milkweed, while females had more neural tissue. It isn’t clear just what that means for their fitness or their ability to survive long enough to reproduce, but it is certain that the extra sodium in their environment does make a difference for the development of their anatomy and physiology.

Finally, the researchers verified that there was such a thing as too much sodium. They reared a different species, cabbage white butterflies, on artificial diets containing low, medium, or high levels of sodium. While there were no survival differences between the butterflies that ate the low or medium sodium diets, those who were fed the high-sodium food were much more likely to die young.

The problem is that these animals evolved to seek out sodium, but now find themselves in an environment rife with it. What are the implications? Ecologists and wildlife biologists have already documented altered foraging behavior in other species, thanks to road salt. Ants that live closer to salted roads spend less time seeking out sodium than those whose colonies are farther from roads. Moose also prefer to drink from roadside ponds, possibly because of salt runoff. The same could be true for butterflies and moths, which can detect sodium concentration through their chemoreceptors. Could the drive to acquire sodium lead butterflies and other wildlife to prefer roadside habitats? If so, that could come with additional problems, such as increased likelihood of death because of car collisions (which is already the case for moose), or for selection on flight behaviors to avoid cars (which has been observed for birds). Increased preference for the salty roadside would also increase the butterflies’ exposure to the toxins in car exhaust.

“Our results highlight the need for more comprehensive descriptions of how nutrition is changing in the face of [human behavior] and the diverse responses…to such changes,” writes Snell-Rood. Combine the effects of atypical sodium intake on animals’ anatomy and physiology with the side effects of living near roads, and its plain to see – yet again – that human behavior has the potential to dramatically affect the evolution and development of the wildlife with which we must coexist. – Jason G. Goldman | 11 June 2014